The present invention relates to an improvement of the system disclosed in U.S. Pat. No. 3,786,337; 01/15/74 for THYRISTORS FOR EFFECTING TAP CHANGING ON TRANSFORMERS AND INCLUDING CURRENT LIMITING RESISTORS AND STANDBY TRIGGER SYSTEM, assigned to the same assignee as the present invention.
Prior art tap-changing systems include a transformer winding having at least two taps. Either of the two taps may be connected selectively to an outgoing current-carrying line. The connection between the taps and the aforementioned line is normally effected by current-carrying contacts forming part of a transfer switch. The latter includes also disconnect contacts. The current-carrying contacts can be shunted by a pair of thyristor networks which are arranged in series with the aforementioned disconnect contacts. Each thyristor network includes a pair of inverse parallel connected thyristors.
In the stationary state, i.e. when the load current is derived only from one of two contiguous taps of a tapped transformer winding and the other tap does not supply any load current, one of two pairs of current-carrying contacts of the transfer switch carries the entire load current, and no other contact of the transfer switch is current carrying. In that stationary state the constituent thyristors of the two thyristor networks are not triggered and, therefore, non-conductive.
A tap-change involves the following steps in the order stated below:
One of the two pairs of disconnect contacts is closed -- if it had not been closed previously -- to prepare a current path shunting the pair of engaged current-carrying contacts which at the time is carrying the load current. That current path includes one of the two thyristor networks, or the first thyristor network. After triggering of the constituent thyristors of the first thyristor network the load current flows through that thyristor network and the closed disconnect contacts arranged in series with it. Hence the current-carrying contacts by which the load current had heretofore been carried may safely be separated; and are separated. Thereupon the disconnect contacts arranged in series with the other or second thyristor network are closed and the constituent thyristors of said second thyristor network triggered, and no further trigger pulses are applied to constituent thyristors of said first thyristor network. As a result, both thyristor networks will be conductive during a short period of time, and the first thyristor network becomes non-conductive following the first zero of the current wave. Now the entire load current is carried by the second thyristor network and by the disconnect contacts that are arranged in series with it. The last step in the process of a tap-change consists in shunting the second thyristor network by closing a pair of current-carrying contacts of the transfer switch, and thereafter deactivating the second thyristor network by interrupting the supply of trigger pulses to it. Now a new stationary state is reached, i.e. the entire load current is derived from the tap that heretofore had been inactive, and the tap that heretofore had supplied the entire load current is now inactive.
The required sequence of operations may be achieved with special transfer switches which are well known in the art. This sequence of operations may also be achieved with any Jansen type transfer switch for tap-changing regulating transformers by resorting to the auxiliary control means disclosed and claimed in U.S. Pat. No. 3,710,232 to M. Matzl, Jan. 9, 1973 for LOGIC-CONTROLLED THYRISTOR SYSTEM FOR PERFORMING TAP-CHANGING OPERATIONS.
What has been described above is a faultless operation of the kind of system under consideration. Such systems may, however, malfunction under certain circumstances.
One of the most dangerous situations results when by malfunctioning of one part or another the thyristors of both thyristor networks are triggered simultaneously. This results in a solid short-circuit of that section of the tapped transformer winding which is connected to the two simultaneously triggered thyristor networks.
The above situation resulting from malfunctioning of the trigger means may be remedied by arranging in series with each of the two thyristor networks a current-limiting resistor which is shunted by an electric fuse. In such a system simultaneous triggering of the thyristors in both thyristor networks results in blowing of the shunt fuse across one or across both current-limiting resistors. These resistors then limit the fault current to a relatively low level. This makes it possible to continue operation of the faulted system for some time during which the fault may be detected and repaired, and the blown fuse, or fuses, replaced.
The constituent thyristors of one, or the other, or of both thyristor networks may not be triggered at the time when they should be triggered.
Considering the case that no trigger pulses are supplied to the thyristors of the first thyristor network supposed to shunt the then engaged current-carrying contacts at the beginning of a tap-changing operation. As a result, an arc is drawn between the current-carrying contacts when they are caused to part. The current-carrying contacts are not designed to withstand arcing and are, therefore, damaged by the arc which is drawn between them. A tap-changing operation involving arcing between parting current-changing contacts may also result in a complete destruction of an on load tap-changing regulating transformer.
The circuitry disclosed and claimed in the above referred-to U.S. Pat. No. 3,786,337 provides means for precluding damage to a tap-changing system of the kind under consideration in case of a failure of the thyristor triggering means and consequent current commutation failure. This is achieved by the provision of standby, or auxiliary, thyristor trigger means under the control of relay means which are energized by means of auxiliary switch means tied to, and jointly operative with, the parting current-carrying contacts of the transfer switch in case that the load current is not commutated from the current path including current-carrying contacts to a current path including a thyristor network.
The invention to which U.S. Pat. No. 3,786,337 relates is thus based on the reasoning that it is necessary to determine initially whether a thyristor network is ready to commutate the load current from the current path including current-carrying contacts which are about to part, or which have already parted, and that the further operation of the system should be made dependent upon the outcome of such initial determination. In case that the aforementioned thyristor network is not ready for its commutating or current-carrying duty it should be triggered by stand-by, or auxiliary, trigger means.
The present invention relates to an improvement of the system of U.S. Pat. No. 3,786,337 and more particularly to an improvement of the embodiment thereof shown in FIG. 3 of the above patent.
As will be shown below in detail a tap-changing operation in the above prior art system may not be properly performed under certain small load-current conditions. It is the prime object of the present invention to provide a novel tap-changing system not subject to small-load current limitations under consideration.